Increasing the exposure latitude of photographic emulsions by sensitizing



United States Patent Burt H. Carroll, Rochester, N. Y., assignor toEastman Kodak Company, Rochester, N. Y., a corporation of New Jersey NoDrawing. Application December 14, 1950, Serial No. 200,865

1 Claim. (Cl. 957) This invention relates to photography andparticularly to a method of sensitizing silver halide emulsions toincrease latitude.

Latitude as used in photographic terminology is the distance along theexposure axis of the characteristic curve in which an increase inexposure produces a substantially uniform increase in density, that is,it is the projected distance along the exposure axis of the straightline portion of the characteristic curve (Mees, Theory of PhotographicProcess .(1942) page 701). In the present case, latitudewas measured asthe distance along the exposure axis between the points where the gammaof the characteristic curve was 0.3 in the toe and 0.6 in the shoulder.

Total scale is defined as the distance along the exposure axis of thecharacteristic curve, between the extreme ends, that is, the toe-andshoulder of the characteristic curve. This includesreg'ions of underandover-exposure (Mees, supra, page 703).

Speed of the photographic material .is expressed for present purposes asthe reciprocal of the inertia of the material, that is,

where k is a constant and i is the inertia.

Gamma is the tangent of the angle between the straight line portion ofthe characteristic curve and the exposure axis (Mees, supra, page 700).

It is well known that silver halide emulsions of difierent speeds maybeblended to increase latitude (Neblette, Brehm and Priest, ElementaryPhotography (1942), pages 190 and 191). "Emulsions of different speedsmay also be coated in separate layers'forthesamepurpose. Thisarrangement always has the probability that development rates oftheemulsions will'not be thesame so that the characteristic curve will bedistorted except under a singledevelopment condition.

It is therefore an object of the presentinvention to provide a methodfor increasingthe latitude ofa-photographic emulsion. A further objectis to provide a method for increasing latitude without seriouslychanging the gamma orotherwise distorting the characteristic curve.Other objects will appear from the following description of myinvention.

These. objects are accomplished by sensitizing diiferent portions of anemulsion with difierent optical sensitizers for the same spectral regionand then mixing the portions so sensitized.

-In general, my method of increasing latitude comprises 1 sensitizing asilver halide emulsion, for example, a silver bromide or silverchlorobromide emulsion, withean optical sensitizing dye, sensitizinganother portion of the same emulsion with a different opticalsensitizing dye for the same spectral region but which gives a differentspeed,

2,703,282 Patented Mar. 1, 1955 about 25% of any one emulsion. It isalso possible to coatthe emulsions as separate layers rather than mixingthem and coating as a single layer.

My method of sensitizing differs from super-sensitizing in that theoptical sensitizers may, and normally will, each sensitize as though theother were not present. The existence of super-sensitizing action of oneor the other would not, however, prevent the use of my method; it willvery rarely occur, however, since almost all of each of the dyes mustremain on the grains to which they were originally added. My methoddiffers from emulsions designed to give variable contrast (U. S. Patent2,384,598) in that the two sensitizers used must sensitize for the samespectral .region.

It is obvious that the optical sensitizing dye used according to myinvention should be relatively nondifrusing since the dyes must beretained on the silver halide grains to .whichthey are originallyattached. Dyes of the following classes are suitable for use accordingto my invention.

(1) Carbocyanines derived from fl-naphthothiazole substituted in thecentral position of the chain by alkyl or aryl groups (Brooker U. S.Patent 1,846,301). include Corresponding dyes from ,B-naphthothiazolenuclei substituted in the nucleus by halogen, alkyl or alkoxy may alsobe used. The alkyl groups on the 3-positions may be replaced byhydroxyalkyl or carboxyalkyl, and the aryl group in the 9-position maybe substituted by alkyl, halogen or alkoxy. These are red sensitizers.

(2) Carbocyanines derived from substituted benzothiazole orbenzselenazole substituted in the central position of the chain by alkylor aryl groups, for example, 3 ,3 ,9-triethyl-5 ,5-dichloroselenacarbocyanine bromide (made by the method of White U. 8.1,990,681 using 5-chloro-2-methyl-benzoselenazole instead of Z-methylbenzoselenazole). Theseare re'd sensitizers.

(3) 3,3 dimethyl -'8,10 aryloxythiacarboxyanines and oxacarbocyanines(Brooker and .White U. S. Patent 2,478,366). These include3;3'-dimethyl-8,IO-m-toloxythiacarbocyanine bromide and-3,3'-dimethyl-8,l0-m-toloxyoxacarbocyanine bromide. These are red andgreen sensitizers, respectively.

(4) Carbocyanines derived from S-phenyl benzoxazole and S-(p-tolyl)benzoxazole substituted in the central position of the chain by alkyl oraryl groups, for example, 3,3',9'-triethyl-5 ,5 -diphenyloxacarbocyanine bromide (See British Patent 496,116). These are greensensitizers.

(5) Complex merocyanines of U. S. Patent 2,454,629.

(6) Merocyanines derived from l-benzothiazolyl-S- pyrazolone, such asthose disclosed in Brooker U. S. Patent 2,211,762.

The optical sensitizing dyes are used according to my invention at aconcentration of from about 4 to 50 milligrams of sensitizing dye perliter of emulsion containing about 0.25 gram mole of silver halide perliter of emulsion, the preferred concentration varying with the dye andwith the grain size of the emulsion in accordance with normalsensitizing practice. An excess of dye is to be avoided because .it.leaves unabsorbed dye which diffuses to grains carrying the other dye.

The sensitizing dyes of groups 1 to 4 above, have the followingstructure:

These where Z and Z=the atoms necessary to complete a B-naphthothiazole,5-aryl benzoxazole or 5-aryl benzothiazole ring,

R and R1==alky1,

R3=hydrogen, alkyl or aryl,

R2 and R4=hydrogen and aryloxy, Ra being hydrogen when R2 and R4 arearyloxy, and R2 and R4 being hydrogen when R3 is alkyl or aryl.

The following table illustrates the increase in latitude which may beaccomplished by blending suitable sensitized emulsions according to myinvention.

My invention will now be described by reference to the followingspecific examples.

Example I A gelatino-silver chlorobromide emulsion containing gram moleof silver chlorobromide per liter was divided into two equal portions.To one portion there was added an alcoholic solution of the redsensitizer 3,3'-dimethyl-9-ethyl-4,5,4',5'-dibenzthiacarbocyaninechloride, 0.058 gram of sensitizer per mole of silver halide being used.The emulsion was digested for about five minutes at 50 C. with the dye.A portion of this emulsion was cooled to the coating temperature ofapproximately 38 C.

coated and tested. Characteristics to red light exposure were found tobe 10/i Speed 130 Gamma 3.40 Latitude 19 To the second portion ofemulsion there was added an alcoholic solution of the red sensitizer3,3'-dimethyl-9- phenyl-4,5,4,5-dibenzthiacarbocyanine chloride, 0.058gram of sensitizer being added per mole of silver halide. The emulsionwas digested at 50 C. and a portion was cooled to 38 C., coated andtested. Characteristics to red light exposure were found to be 10/ iSpeed Gamma 3.70 Latitude 10 The remaining portions of both emulsionswere cooled to the coating temperature of 38 C. and blended and coatedwithout delay. The characteristics of the blend on exposure to red lightwere found to be 10/i Speed 113 Gamma 2.35 Latitude 55 or more Example 2Two portions of the chlorobromide emulsion used in Example 1 wereseparately sensitized as described in Example 1 with different opticalsensitizing dyes. One-half of the emulsion was sensitized with the greensensitizer 3,3,9 triethyl 5,5 diphenyl oxacarbocyaniue bromide, 0.078gram of sensitizer being used per mole of silver halide and the otherhalf of the emulsion was sensitized with the green sensitizer l (2benzothiazolyl) 3 methyl 4 [l methyl 2(1) B naphthothiazolylidene 1ethylethylidene] 5 pyrazolone (see U. S. Patent 2,231,659, Example 15),0.029 gram of sensitizer being used per mole of silver halide.

Upon coating and testing the emulsions separately, the portionsensitized with the first-mentioned dye when exposed through a WrattenFilter No. 1.2 transmitting light of about 500 to about 700 millimicronwave length was found to have a speed of 63, gamma of 3.46 and latitudeof 12.5. The portion sensitized with the second-mentioned dye whenexposed and processed in the same way was found to have a speed of 19,gamma of 3.82 and latitude of about 16. After blending the emulsions,the blend when exposed and processed in the same way was found to have aspeed of 54, gamma of 2.45 and latitude of or more.

Example 3 A portion of the emulsion sensitized to red light as describedin Example was mixed with an equal amount of the emulsion sensitized togreen light as described in Example 2, and the mixture was coateddirectly after mixing.

After drying, the coating was exposed to a color chart and was processedas follows, all of the solutions being at 68 F.:

(l) Prehardened in the following solution for 3 minutes:

Sodium hexametaphosphate grams 0.5 Sodium bisulfite do 10 Formaldehyde(40% solution) cc 27 Sodium carbonate grams 10 Sodium sulfate do..Potassium bromide do 2.5

Water to 1 liter.

(2) Washed for four minutes. (3) Developed in a black and whitedeveloper of the following composition for 8 minutes:

Grams Sodium hexametaphosphate Sodium sulfide 40 N-methyl-p-aminophenolsulfate 5.5 Hydroquinone 2.2 Sodium metaborate 25 Sodium thiocyanate0.45 Potassium bromide 4.5 6-nitrobenzimidazole .02

Water to 1 liter.

(4) Washed for 10 minutes.

(5) Reexposed to red light through a Wratten No. 70 filter.

(6) Developed in a cyan developer of the following composition for 15minutes:

Grams Sodium hexametaphosphate 0.5 Sodium sulfite 52-amino-5-diethylaminotoluene hydrochloridenu 2.8 Sodium carbonate 15Potassium bromide 1.5 6-nitrobenzimidazole 0.15

2,4-dichloro-5-p-toluene sulfonamino-l-naphth'oL 0.4 Sodium hydroxide 2.Water to 1 liter.

Grams Sodium hexametaphosphate 0.5 Sodium sulfite 5 2-amino-5-diethyltoluene hydrochloride 2 Sodium carbonate 20 Potassium bromide 16-nitrobenzimidazole .01 Z-cyanoacetyl coumarone 2.5 Sodium hydroxide 2Water to 1 liter.

l2) Washed for 10 minutes. (13) Treated for four minutes in thefollowing ferrocyanide bath:

Grams Sodium hexametaphosphate 1 Potassium ferricyanide 80 Potassiumbromide 40 Water to 1 liter.

(14) Fixed fortwo minutes in the following bath:

. Grams Sodium hexametaphosphate 10 Sodium sulfite 10 Sodium thiosulfate320 Water to 1 liter.

( 15) Washed for 20 minutes. The image thus produced was satisfactoryfrom the standpoint of color separation and image quality.

-It is apparent that my method has resulted in a substantial increase inthe latitude of the blended emulsion.

This method is useful in producing either black and white or coloremulsions. It is especially useful in mixed grain processes of colorphotography where two or more batches of silver halide sensitive tospectral regions are incorporated in the same emulsion layer. In suchcases it is desirable to have sufficient latitude in each of thedifierent spectral regions so that the exposure light in the variousspectral regions can be balanced to produce images having similarcharacteristic curves. The advantage in freedom from ditferences indevelopment rate of the blended emulsions is obvious. My method alsoprovides a blending procedure which is uniformly reproduceable fromemulsion to emulsion.

Where I refer in the claims'to sensitizing for a spectral region oflonger wave length than that which is appreciably absorbed by saidemulsion, I mean sensitizing for longer wave length than about 500millimicrons in the case of a silver bromoiodide emulsion, about 450millimicrons in the case of a pure silver bromide emulsion, or about 400millimicrons in the case of a pure silver chloride emulsion; that is,the sensitizing should be for a longer wave length than the naturalsensitivity or absorption of the emulsion.

It will be understood that my invention is to be taken asllirilited onlyby the scope of the appended claim.

c aim:

The method of increasing the exposure latitude of a gelatino-silverhalide emulsion, which comprises dividing said emulsion into twoapproximately equal parts, sensitizing one part of said emulsion to redlight with a 3,3- dimethyl 9 ethyl 4,5,4,5' dibenzthiacarbocyanine salt,sensitizing the other part of said emulsion to red light with a'3,3-dimethyl-9-phenyl-4,5,4',5'-dibenzthiacanbo'cyanine salt, and mixingthe parts so sensitized.

References Cited in the file of this patent OTHER REFERENCES Wall,Photographic Emulsions (1929), publ. by American Phot. Pub. Co., Boston;pp. -137.

Clerc, Photographic Theory and Practice, 2nd ed. (1931), publ. by SirIsaac Pitman & Sons, Ltd., N. Y., p.

